The driver of Globe cab 149 is black, slim, of medium height. The standard mug shot and identification card also gives his name: Bernard Sweetney. What it does not say is that Sweetney is one of Washington's better-known working jazz musicians - work that frequently means driving a cab.

Sweetney, 36, is a drummer and vibraphonist notable as the backup percussionist of virtuoso reed man Andrew White. For four years he was Roberta Flack's drummer, before that was on the road with Shirley Horn, and since 1973 has been a regular at Mr. Henry's Capitol Hill, the Rogue and Jar and the Top o'Foolery. This month he's playing Wednesday through Saturdays with Marshall Hawkins at Jazz Uptown.

Sweetney has a degree in art education from University of Maryland, Eastern Shore. He also taught for two years in Petersburg, Va., before returning here to pursue a master's degree at Howard. But in his spare time he was studying music, too - and music won.

In the 11 years since his student days, Sweetney has gained a measure of fame among jazz aficionados, as well as many hours behind the wheel. The number of club dates he gets determines the amount he drives. If he's playing two eveings, he might hack five out of seven days that week; if he's playing five evenings, he might not work the taxi at all.

On stage, the cab driver is nowhere to be seen. Sweetney the jazz musician - on vibes this night, leading his own group - wears a red stocking cap, blue denim pants and jacket and an orange ascot. He is bowed over his instrument one moment, rises up the next, as smiles and grimaces pass alternately over his face. Sometimes he stops for a minute, eyes wide, with a look of surprise, vulnerability, innocence. He starts playing again, fast, so that his red mallet and his white mallet blend together in a flurry of pink.

During a break, he's willing to try to describe what it is about his music that makes the irregular work, the part-time life as a taxi driver, and the dust gathering on his college degree, worth it to him.

"I like music that sings, that moves you," he said, "music that lingers." His solos that night had had a traditional feeling, and he concedes that he doesn't play "the free thing" kind of jazz. For instance, when he plays the vibes he improvises strictly within the chord structure of the piece.

He says he is a Capricorn, and like the goat of his sign, stays on his path, picking his way up the mountain. But he tries not to take paths that have already been followed. "Each person is unique," he remarks, "so each musician has his own thing to say." Sweetney says that he could never play like Bobby Hutcherson or Milt Jackson, the big-name jazz vibraphonists. "I can only play like Bernard Sweetney."

Sweetney's constant awareness of where he is harmonically in his solos does not prevent him from letting himself be transported by the music: "Every tune you use has to kill you. That's the only way you can lose yourself and really groove.When you're out there, you're not aware that you're hungry or that your feet are tired. The music is all that's happening. It's like you're in the vacuum in the inside of a cone that's spinning round and round."

Sweetney can be out there both on vibes and on drums. Equal facility on these two instruments is rare in jazz; the vibes are patently harmonic, the drums percussive. A drummer who leads a group needs to be able to hear, a few bars in advance of the soloist, the chords the soloist will be playing over. The only way a drummer can have such a feeling for harmony is by playing a harmonic instrument. Thus, drummers Tony Williams and Philly Joe Jones, who lead their own groups, both play the piano. Sweetney gets his harmonic knowledge by playing the vibes.

How can a jazz musician always know what chords he is playing over as he improvises, especially when the chord changes are complex and the tempo fast?

"Simple," Sweetney says. "I know the changes becauses I think about music 90 per cent of my time. During the day I hear tunes in my mind, hear the different things I can do with him. That's what you've got to do if you're a real musician; you've got to dedicate yourself to the music, put it up front. You can't be buying big cars and fancy clothes because those things don't relte to what you're dealing with. And that is the truth." Still, a musician needs diversion, and this diversion has to complement his music. Lately he has been changing pace by painting and taking pictures. Pictures of what? "Derelicts, man, derelicts. They've got a lot to say."

The dedication necessary to be a jazz musician demands that Sweetney excise from his life whatever does not relate to his art; it also forces him to take more responsibility than most people take. "When you get down, man, you have to take care of your own Joneses. And you have got to be self-motivated: Nobody tells you what you have to practice, or for how long. You only have yourself to be responsible to."

A basic way Sweetney handles this responsibility is by practicing six to seven hours a day - time that does not include the hours spent playing an evening gig. During his practice Sweetney works with material both from jazz and from European classical music; thus, lately, he has been working on some Chopin and on a Bach piano concerto.

Sweetney likes to practice in the morning. "That's the best part of the day," he says. "At 10:30, man, I be doing some studying and practicing. But by 6:30, when everyone else be coming home, I be pullin up my covers."

And four hours later, when everyone else is going to bed, Bernard Sweetney has only just begun his evening's work. This graveyard shift does make his life different from that of most people. But what really sets him apart is his commitment: "Every person is given a certain thing to do in his life. Some cats are supposed to be street sweepers, some supposed to be lawyers. When you grow up you've got to decide what your particular thing to do is, you've to have that big meeting with yourself."

Part of that decision is whether one is primarily out to make lot of money or whether one is determined to do what he is fit to do, even if doing it doesn't bring in much money. Good musicians choose the latter, Sweetney thinks, and make bucks only if they get lucky. But the money isn't really important: "Look at Rubinstein, man. Ninety years old and he's still playing his piano. That cat is doing what he's supposed to be doing."

Most people, though, are not like Rubenstein, and do not do what they are born to do; instead, "They fall into a mold that someone else sets for them," Sweetney observes, "and then they don't want to hear that they're doing the wrong thing. An artist threatens those people because that artist is doing what they have always secretly wanted to do."

One evening, at a club where he was performing, Sweetney was looking tired and discouraged.Someone told him so, and asked him why. "Been hitting it," he replied. "Working on a Bach concerto all day long." Sweetney looked down, shook his head. "That old dude is knocking me out."